Thank you so much John ! you are really very kind and helpful, thanks for the tips. Tonight I will try to locate the plug and follow your instructions.

Here are the cover pictures. There are two close-up pictures and one from inside the cabin (right side). It is difficult to remove the rear seats, so the cover is blocked by the rear right seat.
Do you know what this round cover is for?

Thanks in advance.

Yes, this is the same drawing that I saw. The plug is the number 2 and the gasket is the number 3.

Thanks John for your fast response. May you give me some information about the cover? . This is because I have seen a drawing of a rear "final Drive" showing in the top, left side, a plug with a gasket, very near to the circular cover mentioned before.

I want to change the rear differential oil in my 2004 S60 R. I have seen a plug in the lower left side, it may be the drain, but I can't see any refilling plug because it's impossible to see anything in the top side. Behind the right rear seat there is a circular cover about 20 cm (8") in diame...